# SNR simulation and "desired output signal"

I'm simulating an amplifier and want to know if it is possible to define its SNR. A simulation in LTSpice shows that the circuit generates 42nV/Hz, but this value is independent from the input/output voltage. Obviously the signal-to-noise ratio will vary depending on the output signal. Is there maybe a standardized output voltage in relation to which SNR is usually expressed? I see that audio equipment, like preamplifiers and audio recorders, always indicate their SNR in dB. But in relation to what output voltage?

Also, output voltage will depend on input voltage and the overall noise of a circuit should vary according to it, I guess. ie. thermal noise from resistors should change with input voltage. Does Spice not take this into account? Probably I'm falling into misconceptions or I am not simulating this properly.

Spice directive: .noise V(n001) V1 dec 10 30 15k

• An amplifier by itself does not have an SNR, as such. The output of an an amplifier does, and it's simply the ratio (dB) of the signal power to the noise power. So if the input signal is noise free, then the SNR of the output will vary with the level of the input signal, assuming the gain is constant. Nov 24, 2023 at 21:33

I see that audio equipment, like preamplifiers and audio recorders, always indicate their SNR in dB. But in relation to what output voltage?

Well, the audio company marketing people will always try to ensure that the most optimum conditions apply and therefore yields the greatest value of SNR possible.

However, SNR is the property of a signal and not a direct property of an amplifier. Look at it this way, if the input had a poor SNR then the output will have a slightly poorer SNR.

Maybe you should look for the term noise factor: This tells you how much an amplifier degrades a signal and is based on the worsening SNR: -

Noise factor = $$\\dfrac{SNR_{INPUT}}{SNR_{OUTPUT}}\$$

Maybe this wiki page might be useful to you: Noise figure (NF) and noise factor (F)

• I understand what you're saying about noise factor and the related noise figure. But I don't think I've ever seen those parameters used when describing audio equipment. Nov 24, 2023 at 22:12
• I see that transistors use noise figures expressed in dB, under explicit test conditions. How do those dB translate into voltage? If the input signal is ie. 100mV, the transistor's NF is 2dB, how do those 2dB translate into injected (nano)voltage? Sorry if I'm asking too much here, it's difficult to grasp noise and dB theory without guidance. Nov 25, 2023 at 15:39
• @domingo the noise figure tells you how much the amplifier reduces SNR but like any circuit, it's not much use knowing that if they don't state the signal input or output levels. So, in your comment you say 100 mV but you haven't stated the input SNR so, I can't give an answer. Nov 25, 2023 at 15:54
• If the noise figure is 2 dB with a 100 mV input and the input SNR is 80 dB then the output SNR is 78 dB @Domingo Nov 25, 2023 at 19:36
• 80 dB in voltage is a real ratio of 10,000:1 so, the noise will be 10 uV @Domingo <-- I stopped checking numbers at that point except to say that bringing in noise density numbers is wrong without specifying the band width of your amplifier and anyway, it doesn't bring anything to the party when you already have a noise figure. Don't allow your overall question to become a chameleon (changing colour too much). In other words, if you want to ask about noise spectral densities then that should be an entirely new question. Nov 25, 2023 at 20:32

Audio equipment have nominal output levels. Recorders, amps and mixing devices have also nominal input levels. It's either dBV or dBm to the nominal resistive load. Use that nominal level as your reference to calculate the SNR. If the manufacturer specifies the SNR of an audio amp or other device which has an input and output the reference level is that specified nominal level.

Microphones which have integrated preamps have their noise specs as equivalent noise sound levels (dB over the hearing treshold).

• This is the way I remember things, when I was into audio gear. Nov 24, 2023 at 22:13
• What would be the criteria to choose the nominal voltage levels? The highest input voltage an amp can withstand before reaching a certain THD value on the output? Or the maximum output voltage before clipping what comes after ie. full-scale range if an ADC is involved? Nov 25, 2023 at 14:59